Quadrature modulator and demodulator

ABSTRACT

Quadrature demodulator ( 10 ) and quadrature modulator ( 30 ) which comprise a first oscillator ( 11 ) and a second oscillator ( 12 ), a separate excitation signal being fed to the first osciallator ( 11 ) and second oscillator ( 12 ) in order to determine the point in time at which switching between two stable states lakes place, and the quadrature demodulator ( 10 ) and quadrature modulator ( 30 ) further comprise excitation means ( 17, 18 ). In the quadrature demodulator ( 10 ), an input signal S i (t) is fed, by means of which signal a parameter of one of the elements of the first and the second oscillator ( 11, 12 ) is influenced and a set of quadrature Output signals I o , Q n  is produced. In the quadrature modulator, a first quadrature signal and a second quadrature signal are fed to the quadrature modulator ( 30 ), by means of which signals a parameter of one of the elements of the first and the second oscillator ( 11, 12 ) is influenced, and the quadrature modulator ( 30 ) further comprises summing means ( 31 ) in order to produce a modulated output signal S o (t).

[0001] The present invention relates to a quadrature modulator anddemodulator comprising a linked oscillator circuit which contains afirst oscillator and a second oscillator, wherein the first oscillatorcomprises a feedback amplifier and an integrator, an output of thefeed-back amplifier is connected to an input of the integrator, and anoutput of the integrator is connected to an input of the feedbackamplifier, the first oscillator has two stable states which alternatewith one another in an oscillation period and a non-stable orregenerative state between the two stable stats, and a separateexcitation signal is fed to the first oscillator in order to determinethe point in time at which switching between the two stable states takesplace, the second oscillator is identical to the first oscillator, andthe quadrature demodulator further comprises first and second excitationmeans, respectively, which are arranged to derive an excitation signalfor the second and the first oscillator, respectively, from an outputsignal from the first and the second oscillator, respectively.

[0002] Quadrature modulators and/or quadrature demodulators are neededfor a multiplicity of circuits, such as in modern communicationsequipment (GSM telephones, DECT telephones, cable modems, etc.).

[0003] In existing quadrature modulators and demodulators use is made ofseparate mixing circuits or modulators and an oscillator. A mismatch inor between the mixing circuits leads to immediate disruption of thequadrature relationship between the in-phase signal I and the quadraturesignal Q and thus to mutual crosstalk. The mixing circuits can be of the(double) balanced type or can be implemented by means of ananalogue/digital converter or a circuit with switched capacitors. In thecase of the balanced mixing circuits an offset can be reduced only bycalibration. The other types are in general more accurate, but of morecomplex design and in general a digital correction can be employed. Theoscillator for generating quadrature signals cans for example, beimplemented by making use of a frequency divider or a(frequency-dependent) phase shift circuit However, oscillators of thesetypes do not have an integral quadrature correction mechanism. Anothertype of oscillator tat can be employed makes use of a phase locked loop(PLL). In this case the two quadrature components do not have the samefrequency characteristics ad the accuracy of the phase detector is thedetermining factor for the operation of the oscillator.

[0004] U.S. Pat. No. 5,939,951 discloses, for example, a method anddevice for modulating and demodulating a signal. The device comprisestwo feedback loops for generating output signal components from an inputsignal. Each loop comprises a voltage-controlled oscillator and acomparator for generating a control signal. An operating signal isgenerated in each loop, the signals from the two loops having aquadrature phase relationship.

[0005] The aim of the present application is to provide a quadraturemotor and demodulator which does not display the abovemetioneddisadvantages and maintains an accurate quadrature relationship.

[0006] Said aim is achieved by a quadrature demodulator of the typedescribed in the pre-amble, wherein an input signal S_(i)(t) is fed tothe quadrature demodulator by means of which a parameter of one of theelements of the first and the second oscillator is influenced, theelements comprising the feedback amplifier, integrator or excitationmeans, and a set of quadrature output signals I_(o), Q_(o) is fed to theoutputs of the respective one element that is associated with theinfluenced parameter.

[0007] By integrating the mixing circuits (multipliers) of thequadrature demodulator in a quadrature oscillator which generates tworeference signals having a highly accurate quadrature relationship, ahighly accurate quadrature demodulator is provided. The mixing circuitsare implemented by varying a switching parameter of the quadratureoscillator at the speed of the signal to be demodulated In this case thequadrature control mechanism (feedback) of the oscillator alsosuppresses the effects of any mismatch of the mixing circuits to thequadrature relationship.

[0008] It is noted that U.S. Pat. No. 5,233,315 discloses a coupledregenerative oscillator circuit In this oscillator circuit an accuratequadrature phase relationship is maintained by means of a feedback loop.

[0009] In one embodiment the parameter is one of the followingparameters:

[0010] a threshold level (γ) or an output level (δ) of the feedbackamplifier, or an integration constant (α) of the integrator.

[0011] In a preferred embodiment the excitation means comprise a firstand, respectively, second soft limiter circuit associated with the firstand the second oscillator, respectively, and the parameter is theamplification (G) or the limit level (β) of the soft limiter circuit

[0012] In this embodiment influencing the parameters of the soft limitercircuit has no influence on the zero passage detection of the softlimiter circuit, The function of the generation of an excitation signalfor the other oscillator is thus not affected by influencing the oneparameter. The multiplication function is now part of the negativefeedback loop of the quadrature-coupled oscillator. It is advantageousto use the parameter β of the soft limiter circuit as the parameter tobe influenced, because in this case the output signal from the softlimiter circuit is to the parameter β. As a consequence the amplitudesof the (I and Q) quadrature signals also remain the same.

[0013] In a further aspect the present invention relates to a quadraturemodulator comprising a linked oscillator circuit which contains a firstoscillator and a second oscillator, wherein the fist oscillatorcomprises a feedback amplifier and an integrator, an output of thefeedback amplifier is connected to an input of the integrator, and anoutput of the integrator is connected to an input of the feedbackamplifier, the first oscillator has two stable states which alternatewith one another in an oscillation period and a non-stable orregenerative state between the two stable states, and a separateexcitation signal is fed to the first oscillator in order to determinethe point in time at which switching between the two stable states takesplace, the second oscillator is identical to the first oscillator, andthe quadrature modulator further comprises first and second excitationmeans, respectively, which are arranged to derive an excitation signalfor the second and the first oscillator, respectively, from an outputsignal from the first and the second oscillator, respectively,characterized in that a first quadrature signal and a second quadraturesignal are fed to the quadrature modulator, by means of which signals aparameter of one of the respective elements of the first and the secondoscillator is influenced, the elements comprising the feedbackamplifier, integrator or excitation means, and the quadrature modulatorfurther comprises summing means which are connected to the respectiveoutput signals of the one element that is associated with the influencedparameter, in order to form a modulated output signal S_(o)(t).

[0014] In one embodiment the parameter is one of the followingparameters:

[0015] a threshold level (γ) or an output level (δ) of the feedbackamplifier, or an integration constant (α) of the integrator.

[0016] In a preferred embodiment the excitation means comprise a firstand, respectively, second soft limiter circuit associated with the firstand the second oscillator, respectively, and the parameter is theamplification (G) or the limit level (β) of the soft limiter circuit.

[0017] The quadrature modulator according to the present inventionprovides advantages corresponding to those of the quadrature demodulatoraccording to the present invention, as discussed above.

[0018] In yet a further embodiment the quadrature modulator ordemodulator further comprises at least one ether oscillator that isidentical to the first and the second oscillator, and associated furtherexcitation means that are equipped to derive an excitation signal for asubsequent oscillator of the at least one further oscillator from anoutput signal from the at least one further oscillator. This appreciablyexpands the number of possibilities for modulation/demodulation. In thiscase, however, no use is made of an orthogonal (minimal) set of basicwave forms, such as the quadrature signals in the embodiments mentionedabove.

[0019] The present invention will now be explained in more detail on thebasis of a number of examples, with reference to the appended drawings,in which:

[0020]FIG. 1 shows a block diagram of one embodiment of the quadraturedemodulator according to the present invention;

[0021]FIG. 2 shows a block diagram of one embodiment of the quadraturemodulator according to the present invention,

[0022]FIG. 3 shows a diagram of an integrator circuit that forms part ofthe quadrature modulator/demodulator according to FIGS. 1 and 2;

[0023]FIG. 4 shows a diagram of a soft limiter and multiplicationcircuit that forms part of the quadrature modulator/demodulatoraccording to FIGS. 1 and 2;

[0024]FIG. 5 shows a diagram of a summing circuit that forms part of thequadrature modulator/demodulator according to FIGS. 1 and 2; and

[0025]FIG. 6 shows a diagram of a Schmitt trigger circuit that formspart of the quadrature modulator/demodulator according to FIGS. 1 and 2.

[0026]FIG. 1 shows a block diagram of a quadrature demodulator 10according to one embodiment of the present invention. In the quadraturedemodulator 10 in question an oscillator 11, 12 coupled in quadrature isused to generate two periodic signals which have a quadraturerelationship. The negative feedback mechanism accurately maintains thequadrature phase relationship between the two periodic signals. Theoscillator coupled it quadrature comprises a first oscillator 11 and asecond oscillator 12, each of which has a Schmitt trigger circuit 13, 15and an integrator circuit 14, 16. The Schmitt trigger circuit 13, 15 canbe formed by a feedback amplifier and is characterised by a thresholdlevel γ and an output level δ. The integrator circuit 14, 16 can beformed by a capacitor and is characterised by an integration constant α.The Schmitt trigger circuit 13, 15 and integrator circuit 14, 16 arecoupled to one another in a loop, that is to say an input of the Schmitttrigger circuit 13, 15 is connected to an output of the integratorcircuit 14, 16 and an input of the integrator circuit 14, 16 isconnected to an output of the Schmitt trigger circuit 13, 15. A signale_(int)(t), that over time, is generated with an integration constant αby the integration circuit 14, 16. The signal e_(int)(t) is madeperiodic by switching between two states in which α and −α,respectively, are integrated. The Schmitt trigger circuit 13, 15switches between these two states if the output signal from theintegrator circuit 14, 16 exceeds the positive or negative thresholdlevel γ.

[0027] The accurate quadrature relationship is achieved by coupling thetwo oscillators 11, 12 to one another. This coupling provides atransition between two states in one of the oscillators 11, 12 by makinguse of the other oscillator 11, 12 as a reference. This can beimplemented by means of a comparator circuit (not shown), which detectsa zero passage of the output signal from the integrator circuit of theone oscillator 11, 12 and derives an excitation signal therefrom, whichexcitation signal is fed to the Schmitt trigger circuit 13, 15 of theother oscillator 12, 11.

[0028] However, in the embodiment sown in FIG. 1 use is made of a firstand respectively, a second soft limiter circuit 17, 18. Such a circuitis characterised by a variable amplification G in a limited region ofthe input signal and a limit value β (V_(out)=−β for V_(in)<−β,V_(out)=G*V_(in) for −β<V_(in)<β, and V_(out)=β for V_(in)>β). An inputof the first soft limiter circuit 17 is connected to the output of thefirst integration circuit 14, and an output of the first soft limitercircuit 17 is fed via a summing element 20 to the input of the secondSchmitt trigger circuit 15. An input of the second soft limiter circuit18 is connected to the output of the second integration circuit 16, andan output of the second soft limiter circuit 18 is fed via a summingelement 19 to the input of the first Schmitt trigger circuit 13.

[0029] The coupling mechanism between the two oscillators 11, 12 is ableto maintain the quadrature phase relationship if a mismatch existsbetween the two oscillators 11, 12, If the two oscillators 11, 12 arenot yet operating in a fixed quadrature relationship, and one oscillator11, 12 has a greater period than the other, the faster oscillator willwait until the slower oscillator 11, 12 passes through zero before itswitches state and is thus delayed. The slower oscillator 11, 12 willswitch over state sooner because of the faster excitation signal fromthe faster oscillator 11, 12 and will thus be accelerated. Ultimatelythe two oscillators will have the same period and will have an accuratequadrature relationship.

[0030] The behaviour of the two oscillators 11, 12 is determined by fiveparameters, i.e, the integration constant α of the integration circuit14, 16, the limit levels β and the amplification G of the soft limitercircuit 17, 18 and the threshold levels γ and output levels δ of theSchmitt trigger circuit 13, 15. In the present invention theseparameters are used to implement the quadrature demodulator 10. Theparameters can be adjusted to the input signal S_(i)(t) to bedemodulated by incorporating a multiplier, the modulated signal S_(i)(t)being fed to one input and the normal local oscillator signal being fedto another input.

[0031] The possibility of using the parameters β and G of the softlimiter circuit 17, 18 to input the signal S_(i)(t) to be demodulatedinto the circuit is discussed below. It is assumed that the soft limitercircuit 17, 18 is in its linear operating region. Because the circuit isset up in such a way that the Schmitt trigger circuit 13, 15 of the oneoscillator 11, 12 switches over when the other oscillator 11, 12 passesthrough zero, this will always be the case in practice.

[0032] If the parameters are kept constant, the output signale_(slim)(t) from the soft limiter circuit 17, 18 is equal toβGe_(int)(t). If the parameter G is used to input the signal Si(t) to bedemodulated into the oscillator 11, 12, the output signal is equal toβ[G+S_(i)(t)]e_(int)(t). The term βS_(i)(t)e_(int)(t) is the desiredoutput signal I_(o) or Q_(o), and can easily be obtained from the outputsignal from the soft limiter circuit 17, 18 by means of a low passfilter 21, 22. Compared with the situation with constant parameters,this signal is the only term in the output signal from the soft limitercircuit 17, 18. This additional term does not introduce any extra zerocrossings if the amplitude of the signal S_(i)(t) to be demodulated issmaller than the magnitude of the parameter G. Because no additionalzero crossings are introduced, the coupled oscillator 10 will switchbetween the two states in the same way as in the case of constantparameters, as a result of which in this case also the quadrature phaserelationship between the output signals I_(o) and Q_(o) remainsguaranteed.

[0033] If, as an alternative, the signal S_(i)(t) to be demodulated isinput by means of the limit levels β of the soft limiter circuit 17, 18,the output signal is given by e_(slim)(t)=[β+S_(i)(t)]Ge_(int)(t). Inthis case the term S_(i)(t)Ge_(int)(t) is the desired output signalI_(o), Q_(o). A situation arises that is comparable to that in theprevious case: if the amplitude of S_(i)(t) is less than the magnitudeof β no additional zero crossings are created. The oscillator will stillchange state when e_(int)(t) passes through zero.

[0034] In both cases described the same results are achieved with regardto the points in time at which the oscillators 11, 12 switch. Theoscillator frequency and oscillator phase are not changed, and thequadrature relationship of the quadrature output signals I_(o) and Q_(o)is thus maintained. The multiplication function of the signal to bedemodulated and the oscillator signal is now part of the negativefeedback loop of the coupled quadrature oscillator, which can correctfaults that occur.

[0035]FIG. 2 shows a quadrature modulator 30 according to the presentinvention. The construction and mode of operation are in broad termsidentical to those of the quadrature demodulator 10 described withreference to FIG. 1. In this case, however, the quadrature input signalsI_(i) and Q_(i) are fed to the soft limiter circuits 17 and 18,respectively, in order to influence one of the parameters β, G. and theoutput signals from the soft limiter circuit 17, 18 are summed in asumming device 31 and if necessary passed through a bandpass filter 32in order to obtain the modulated RF output signal S_(o)(t).

[0036] In order to implement the quadrature demodulator 10 andquadrature modulator 30 four subcircuits have to be designated, i.e. theintegrators 14, 16, the Schmitt trigger circuits 13, 15, the summingcircuits 19, 20 and the soft limiter circuits 17, 18. The four parts aredescribed in move detail with reference to FIGS. 3 to 6.

[0037]FIG. 3 shows a diagram of a possible implementation of theintegrator 14, 16. The integrator 14, 16 can be implemented by makinguse of the voltage/current relationship of a capacitor C_(int). Thecurrent I_(int) from the current sources can be adjusted with the aid ofthe voltage V_(t), and this also sets the frequency of the oscillator11, 12. The frequency of the oscillator 11, 12 is adjustable between 1MHz and 2 MHz. The differentially switched pair of transistors Q₁, Q₂ isused to switch the current through the capacitor C_(int) between I_(int)and −I_(int) in response to the voltage V_(i1). The resistors R₂, R₃,transistor Q₃ and voltage source V_(cm) form a common mode loop and keepthe common voltage on the collectors of Q₁ and Q₂ equal toV_(be3)+V_(cm). The frequency compensation of the common node loop iseffected with the capacitors C₁ and C₂. The output signal from theintegrator 14, 15 is indicated by V_(o1).

[0038]FIG. 4 shows a diagram of a possible implementation of the softlimiter and multiplication circuit 17, 18. This soft limiter circuit 17,18 implements the multiplication by the signal S_(i)(t) to bedemodulated and the soft-limiting function. The input signal V_(i2) isconnected to the output voltage V_(o1) of the integrator circuit 14, 16and the input signal V_(i3) is connected to the signal S_(i)(t) to bedemodulated. A differential transistor pair Q₄, Q₅ provides the softlimiter function and, together with the transistor Q₆, a mixing circuit.The multiplication of V_(i2) and V_(i3) is present as the output signalI_(o2) of the mixing circuit. The resistors R₃, R₄, and R₅ and thevoltage source V_(cc) are used to adjust the bias of the transistor Q₆.

[0039]FIG. 5 shows a diagram of a possible implementation of the summingcircuit 19, 20. An input I_(i3) is connected to the output of the softlimiter circuit 17, 18 and an input V_(i4) is connected to the outputsignal V_(o1) the integrator. The voltage V_(i4) is converted into acurrent in order to make summing in the current domain possible.Transistors Q₇, Q₈ and resistors R₈, R₉ form a balanced series stage forthe summation. Two resistors R₆ and R₇ are used to convert the sum ofthe input current I_(i3) and the voltage V_(i4) converted into a currentinto an output voltage V_(o3). Voltage source V_(cc) and current sourceI_(bias) are used to bias the transistors Q₇, Q₈.

[0040]FIG. 6 shows a diagram of an implementation of the Schmitt triggercircuit 13, 15. The Schmitt trigger circuit 13, 15 serves to generatetwo internal reference levels [γ, −γ], to compare an input signal withthe two reference levels, to switch an output signal between two statesand to store the current state. The two reference levels [γ, −γ], areimplemented by two voltage sources, which are indicated by V_(ref), andin the diagram shown are equal to a base emitter voltage V_(he)=700 mV.The input signal V_(i5), which originates from the summing circuit 19,20 and corresponds to the output voltage V_(o3) of the summing circuit19, 20, is compared with the two reference voltages+V_(ref), −V_(ref) bymeans of the differential transistor pairs Q₉, Q₁₀ and Q₁₁, Q₁₂,respectively. The differential transistor pairs Q₉, Q₁₀ and Q₁₁, Q₁₂produce currents I_(set) and I_(reset) as output signal. Switchingbetween two states and storage of the current state are implemented bymeans of a circuit which comprises a limiter in a positive feedbacks.The limiter is implemented by means of differential transistor pair Q₁₃,Q₁₄. A positive feedback of the limiter is implemented by means of acurrent/voltage converter, formed by resistors R₁₀ and R₁₁. The positivefeedback ensures that the output signal from the limiter Q₁₃, Q₁₄ hasone of two possible output values. If the signal at the input of thelimiter Q₁₃, Q₁₄, passes through zero, the positive feedback ensuresthat the output signal of the limiter Q₁₃, Q₁₄ switches to the otheroutput value. Furthermore, the Schmitt trigger circuit 13, 15 containsvoltage sources V_(bs) in order to provide the transistors Q₁₃, Q₁₄ witha bias and as a level shift in the common mode. The currents I_(set) andI_(reset) are the input signals for the limiter and are summed with theoutput current from the limiter Q₁₃, Q₁₄. The currents I_(set) andI_(reset) must be greater than the output current from the limiter Q₁₃.Q₁₄ in order to initiate changeover to the other state. The bias of thetransistors Q₉ . . . Q₁₄ is adjusted using a voltage source V_(cc) andcurrent sources I_(bias). The voltage between the respective bases oftransistors Q₁₃, Q₁₄ gives the output voltage V_(o5) of the Schmitttrigger circuit 13, 15. This output voltage V_(o5) is fed as inputvoltage V_(i1) to the integrator 14, 16.

[0041] The Schmitt trigger circuit 13, 15 and the soft limiter circuit17, 18 can be implemented with the aid of the same sort of elements, forexample an amplifier having a non-linear (limiting) transmission. In theexamples described above the elements are implemented with complementaryswitched transistors.

[0042] The quadrature modulator or demodulator can comprise multipleoscillators 11, 12 and associated excitation means, such as the softlimiter circuits 17, 18, This appreciably expands the number ofpossibilities for modulation/demodulation. In this case, however, no useis made of an orthogonal (minimal) set of basic wave forms, such as thequadrature signals in the abovementioned embodiments.

[0043] The present invention has been explained above on the basis of afew illustrative embodiments. It will be clear to those skilled in t heart that variations and other implementations are possible. Thesevariations and other implementations are considered to be covered by thescope of protection that is defined by the appended claims.

1. Quadrature demodulator (10) comprising a linked oscillator circuitwhich contains a first oscillator (11) and a second oscillator (12),wherein the first oscillator (11) comprises a feedback amplifier (13,15) and an integrator (14, 16), an output of the feedback amplifier (13,15) is connected to an input of the integrator (14, 16), and an outputof the integrator (14, 16) is connected to an input of the feedbackamplifier (13, 15), the first oscillator (11) has two stable stateswhich alternate with one another in an oscillation period and anon-stable or regenerative state between the two stable sates, and aseparate excitation signal is fed to the first oscillator (11) in orderto determine the point in time at which switching between the two stablestates takes place, the second oscillator (12) is identical to the firstoscillator (11), and the quadrature demodulator (10) further comprisesfirst and second excitation means (17, 18), respectively, which areequipped to derive an excitation signal for the second and the firstoscillator (12, 11), respectively, from an output signal from the firstand the second oscillator (11, 12), respectively characterised in thatan input signal S_(i)(t) is fed to the quadrature demodulator (10) bymeans of which a parameter of one of the elements of the first and thesecond oscillator (11, 12) is influenced, the elements comprising thefeedback amplifier (13, 15), integrator (14, 16) or excitation means(17, 18), and a set of quadrature output signals I_(o), Q_(o) is fed tothe outputs of the respective one element that is associated with theinfluenced parameter.
 2. Quadrature demodulator according to claim 1,wherein the parameter is one of the following parameters: a thresholdlevel (γ) or an output level (δ) of the feedback amplifier (13, 15), oran integration constant (α) of the integrator (14, 16).
 3. Quadraturedemodulator according to claim 1, wherein the excitation means (17, 18)comprise a first and, respectively, second soft limiter circuitassociated with the first and the second oscillator (11, 12),respectively, and the parameter is the amplification (G) or the limitlevel (β) of the soft limiter circuit (17, 18).
 4. Quadraturedemodulator according to one of claims 1 to 3, wherein said demodulatorfurther comprises at least one further oscillator that is identical tothe first and the second oscillator (11, 12), and associated furtherexcitation means that are equipped to derive an excitation signal for asubsequent oscillator of the at least one further oscillator from anoutput signal from the at least one further oscillator.
 5. Quadraturemodulator (30) comprising a linked oscillator circuit which contains afirst oscillator and a second oscillator (11, 12), wherein the firstoscillator (11) comprises a feedback amplifier (13, 15) and anintegrator (14, 16), an output of the feedback amplifier (13, 15) isconnected to an input of the integrator (14, 16), and an output of theintegrator (14, 16) is connected to an input of the feedback amplifier(13, 15), the first oscillator (11) has two stable states whichalternate with one another in an oscillation period and a non-stable orregenerative state between the two stable flares, and a separateexcitation signal is fed to the first oscillator (11) in order todetermine the point in time at which switching between the two stablestates takes place, the second oscillator (12) is identical to the fistoscillator (11), and the quadrature modulator (30) filter comprisesfirst and second excitation means (17, 18), respectively, which areequipped to derive an excitation signal for the second and the firstoscillator (12, 11), respectively, from an output signal from the firstand the second oscillator (11, 12), respectively, characterised in thata first quadrature signal and a second quadrature signal are fed to thequadrature modulator (30), by means of which signals a parameter of oneof the respective elements of the first and the second oscillator (11,12) is influenced, the elements comprising the feedback amplifier (13,15), integrator (14, 16) or excitation means (17, 18), and thequadrature modulator (30) farther comprises summing means (31) which areconnected to the respective output signals of the one element that isassociated with the influenced parameter, in order to form a modulatedoutput signal S_(o)(t).
 6. Quadrature modular according to claim 5,wherein the parameter is one of the following parameters. a thresholdlevel (γ) or an output level (δ) of the feedback amplifier (13, 15), oran integration constant (α) of the integrator (14, 16).
 7. Quadraturemodulator according to claim 5, wherein the excitation means (17, 18)comprise a first and, respectively, second soft limiter circuitassociated with the and the second oscillator (11, 12), respectively,and the parameter is the amplification (G) or the limit level (β) of thesoft limiter circuit (17, 18).
 8. Quadrature modulator according to oneof claims 5 to 7, wherein said motor further comprises at least onefurther oscillator that is identical to the first and the secondoscillator (11, 12), and associated further excitation means that areequipped to derive an excitation signal for a subsequent oscillator ofthe at least one further oscillator from an output signal from the atleast one further oscillator.